netdev - Re: [PATCH] net: fix race in the receive/select

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Date:	Fri, 26 Jun 2009 10:32:17 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	fbl@...hat.com, nhorman@...hat.com, davem@...hat.com,
	Tejun Heo <htejun@...il.com>
Subject: Re: [PATCH] net: fix race in the receive/select

On Fri, 26 Jun 2009, Oleg Nesterov wrote:

> And if we remove waitqueue_active() in xxx_update(), then lock/unlock is
> not needed too.
>
> If xxx_poll() takes q->lock first, it can safely miss the changes in ->status
> and schedule(): xxx_update() will take q->lock, notice the sleeper and wake
> it up (ok, it will set ->triggered but this doesn't matter).
> 
> If xxx_update() takes q->lock first, xxx_poll() must see the changes in
> status after poll_wait()->unlock(&q->lock) (in fact, after lock, not unlock).

Sure. The snippet above was just to show what typically the code does, not 
a suggestion on how to solve the socket case.
But yeah, the problem in this case is the waitqueue_active() call. Without 
that, the wait queue lock/unlock in poll_wait() and the one in wake_up() 
guarantees the necessary barriers.
Some might argue the costs of the lock/unlock of q->lock, and wonder if 
MBs are a more efficient solution. This is something I'm not going into.
To me, it just looked not right having cross-matching MB in different 
subsystems.

- Davide

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